The traditional method of Internet access from a subscribers home is by way of a dialled up connection to a modem pool at a server operated by an Internet Service Provider. A connection of this kind is set up in the PSTN (Public Switched Telephone Network) and information is transferred in the form of TCP/IP (Transmission Control Protocol/Internet Protocol) packets. The subscriber typically uses a PC (Personal Computer) connected through their telephone line by way of a modem at speeds of around 33 kb/s or possibly up to 56 kb/s in some cases. However, once the connection is set up the telephone line is generally unable to be used for other incoming or outgoing calls. Further, the connection does not make efficient use of PSTN resources in that Internet sessions are relatively long and bursty in nature.
A wide range of systems have been proposed to enable use of a single telephone line for multiple simultaneous data and/or voice related calls. Subscribers to ISDN (Integrated Services Digital Network) can access the Internet using DSL (Digital Subscriber Line) techniques which provide one, two or more B channels. ISDN basic access provides a 2B+D transmission format in which the B and D channels form a TDM (Time Division Multiplexed) signal and operate at speeds of 64 kb/s and 16 kb/s respectively. If one B channel is in use for Internet access another can carry incoming or outgoing telephone calls simultaneously, and both B channels may be used for Internet data when no telephone call is underway. However, ISDN requires an entirely new network solution, and for Internet access generally makes the same relatively inefficient use of PSTN resources as the traditional modem connection.
There is rapid development of other DSL access technology, such as ADSL (Asymmetric Digital Subscriber Line) which provides up to about 9 Mb/s downstream to subscribers and up to about 640 kb/s upstream depending on length of the copper pair. VDSL (Very high rate DSL) provides up to about 20 Mb/s in both directions. These are more expensive and complex than ISDN in that frequency bands above that of the PSTN are generally used, and are less able to operate effectively over long lines. Still other developments for simultaneous use of a single telephone line include Pairgain systems, DSVD (Digital Simultaneous Voice and Data) modems, and IP modems by which data and voice signals are combined in an IP data stream at the subscribers home. Most of these except the IP modem, also make inefficient use of PSTN circuit switching resources.